This study will determine whether growth hormone therapy in prepubertal children with Prader-Willi syndrome positively affects linear height velocity and predicted adult height, body composition, resting energy expenditure and bone density, muscle strength, and indices of self esteem. Prader-Willi Syndrome is now recognized to be associated with a small 15q deletion inherited from the paternal germline or with maternal isdisomy. Infants with Prader-Willi syndrome have small hands and feet and a characteristic facial appearance. After initially failing to thrive, they gain weight rapidly as a result of voracious and uncontrolled appetite. Prader-Willi patients have remarkably decreased body muscle mass, weakness, severe osteoporosis in adulthood, slow linear growth and variable hypogonadtropic hypogonadism. If they are maintained in the thinned state, they have further diminished height velocity and yet still have increased budy fat mass. Previous uncontrolled studies have suggested that treatment with grwoth hormone improves body muscle mass as well as linear growth in children with Prader-Willi syndrome.